Gaseous electric discharge lamp



Nov. 8, 1938. J. H. MITCHELL GASEOUS ELECTRIC DISCHARGE LAMP Filed Nov. 8, 1934 mvragm BY A ORNEY Patented Nov. 8, 1938 I PATENT OFFICE GASEOUS ELECTRIC DISCHARGE LAMP John 11. Mitchell, Rugby, England, assignor to General Electric Company, a corporation of New York Application November 8, 1934,,Serial No. 752,136 In Great Britain November 8, 1933 1 Claim.

The present invention relates to gaseous electric discharge lamps generally and more particularly to such lamps of the type disclosed in the United States Patent Number 1,948,261, granted 5 February 20, 1934, to Victor James Francis and John Walter Ryde. Such lamps consist of an envelope containing mercury and activated cathodes and operate at a mercury vapor pressure approaching that of the atmosphere. A heat conservator is useful in maintaining the envelope at an elevated temperature in order to obtain this vapor pressure and an evacuated envelope inclosing the lamp envelope is useful for this purpose. Such lamps sufierfrom the disadvantage that they are deficient in red and blue rays and therefore the light given by them distorts practically all colours.

The object of this invention is to improve the color and quality of the light emitted by a 20 gaseous electric discharge lamp using high pressure mercury vapour. Another object of the invention is to provide a high pressure, unsaturated vapor lamp which emits light closely approaching daylight. To these ends the invention consists 25 in providing a mixture of vapours in the proper proportions to give the desired result and in a method of introducing the vaporizable materials into the lamp.

In carrying this invention into efiect according 30 to one embodiment I provide as a filling a mixture of mercury and cadmium vapour. This mixture' when in the proper proportions greatly improves the colour distribution without seriously reducing the high efficiency of the lamp. I have demon- 35 strated that a good, well balanced light is obtained by using a vapour pressure of about 62.5% of mercury and 37.5% cadmium but variations in the colour intensity can be obtained by varying the cadmium content, or by substituting anr other substance for the cadmium. The exact quantity of the cadmium introduced depends on the dimensions of the lamp and on the electrical supply conditions, such as the voltage, etc., under which the lamp is operated, but in all cases the I 1 quantity of the mercury-cadmium mixture introduced into the lamp is limited so that the vapour pressure of the lamp always remains unsaturated under normal running conditions, that is, at the operating temperature of the lamp container. According to one method of introducing the mixture of metals into the lamp the cadmium is formed into a wire from which a suitable length is cut oil and this with the required weight of mercury is introduced into the lamp container v:35 through the exhaust tube. Alternatively a cadmium mercury amalgam is formed having the ingredients in the proper proportions for the type of lamp desired. This amalgam is formed into a wire and a portion of wire of the required weight is cut oil and introduced into the lamp 5 container through the exhaust tube. The introduction in the form of a wire besides being convenient makes possible the use of a smaller exhaust tube which simplifies the sealing oif operation and improves the lamp as the possibili- 10 ties of cold spots on-which condensation is likely to occur are removed.

The light given by an electric discharge vapor lamp made according to this invention is a much nearer approach to daylight than that given by a lamp containing only mercury vapour. The relative intensities of the light in diflerent parts of the spectrum is readily varied as stated by varying the relative proportions of the mercury and cadmium. By'inserting the mercury-cadmium mixture as described in measured quantities an unsaturated vapour pressure for normal operation is readily obtained.

In the drawing accompanying and forming part of this specification an embodiment of the invention is illustrated in a front elevational, partly sectional view but as such illustration is primarily for purposes of disclosure it will be understood of course that numerous changes in the form and details of the device may be made by those skilled in the art without departure from the spirit and scope of the invention.

Referring to the drawing the new and novel gaseous electric discharge lamp comprises a glass container I having approximately spherical ends 2 into each of which a pair of leading-in wires 3 are sealed. A thermionic electrode comprising a spiral filament 4 of tungsten, for example, wrapped around a rod or bar 5 of electron emitting material, such as a mixture of barium and 40 calcium oxides, is attached to each pair of leads 3 by spot-welding the filament 4 to said leads 3, for example. Two turns free of the rod 5 are used for preventing the conduction of excessive heat from the electrodes to the seals. The electrodes are mounted close to the spherical ends 2 of the container I and are separated not more than 10 mm. from the wall so that said spherical ends 2 are heated to approximately the same temperature than the straight walls of the container l during the operation of the lamp. Condensation of metal vapor in said spherical ends 2 is thus avoided. .When desired the container ends 2 are provided with a coating of material which reduces the radiation of heat therefrom, such as caps oi thin metal foil.

The container I is mounted in an evacuated envelope 1 (shown partly in section in the drawing) having a stem 8 and a screw base I2 at one end thereof. Current leads 8 are sealed into said stem 8 and are connected to the current leads 3 sealed into said container I. 'Said current leads 9 are connected to resilient wire rings II which in turn are connected to the leads 3. Said rings I3 press against the inner wall of said envelope I and assist in supporting said container I in said envelope I.

Said container I has a' starting gas therein, such as argon, at a pressure of a few mm. and such quantity of mercury and cadmium that these metals are completely vaporized at a term perature lower than the operating temperature of said container I and the vapor pressure in the container I is approximately one atmosphere when the lamp device is operating on a current of from 2 to 3 amperes. The mercury and cadmium are introduced into the container I by the method pointed out heretofore and when the two metals are in the proportions given above the light emitted by the lamp closely approaches daylight. A wire III is connected to one of the leads 9 and extends externally along the container I and is wrapped around the container I to form a conductive ring II adjacent the electrode connected to the other or said leads 9 to facilitate starting of the device.

While I have shown and described and have pointed out in the annexed claim certain novel features of the invention, it will be understood that various omissions, substitutions and changes in the forms and details of the device illustrated and in its use and operation may be made by those skilled in the art without departing from the broad spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:-

The method of introducing a quantity of amalgam into a lamp container which consists in forming said amalgam into a wire of smaller diameter than the exhaust tube or said container and passing said wire through said tube into said container.

JOHN H. m'rcmsLn 

